Process For Manufacturing a Ptfe Filament, and a Ptfe Filament, and a Ptfe Filament Obtained By This Process

ABSTRACT

The present invention consists of an inventive process for manufacturing a PTFE filament ( 7 ) of the type comprising steps of extrusion, and, subsequently, stretching, heating and cutting, and an inventive PTFE filament obtained by this process, are provided. The process of the present invention comprises the following steps prior to extrusion providing a recipient, which preferably consists of a cylinder ( 1 ) of pre-form machine, having rigid side walls; arranging a first mixture (A) containing PTFE filler, and a second mixture (B) containing PTFE, inside the recipient, side by side and aligned with the side walls; and pressing the first and second mixtures in a direction parallel to the side walls to form a billet ( 5 ) in which the first and second mixtures (A, B) have different coefficients of friction. After these steps, the billet ( 5 ) is extruded to form a strand ( 6 ), which is subsequently stretched, heated, rolled and cut by known processes in the art to form the inventive PTFE filament, which comprises one side with a filler and the other side without filler, so that these sides have different coefficients of friction. Also, in a preferred embodiment, the filament presents each side with a different color.

This invention refers to a process of manufacturing apolytetrafluoroethylene (hereinafter called “PTFE”) filament, and thefilament obtained by this process.

Since the development of the material in the U.S. Pat. No. 3,953,566 byGore, flexible fibers made from expanded PTFE have been used for severalpurposes, such as fabric, sewing thread and dental floss. Such fibersare widely used due to the very good physical properties of the PTFEresin, and, furthermore, as they are chemically inert, have excellenthigh and low temperature performance, high resistance to ultravioletradiation and are highly lubricious. U.S. Pat. Nos. 3,953,566 and3,962,153 disclose processes for producing highly porous materials fromPTFE that result in very high strength products. These patents disclosehow strands made of this polymer are produced by paste formingtechniques, where the polymer is converted into a paste and shaped intoa strand, which is then expanded by stretching in one or more directionsunder certain conditions so that it becomes much more porous andstronger. This phenomenon of expansion accompanied by an increase instrength occurs with certain preferred PTFE resins and within preferredranges of stretching rate and preferred temperature ranges. Accordingly,most of the products are obtained to when expansion is carried out athigh temperatures, preferably within the range of 35° C. to 327° C.

In addition, it was found that some resins are much more suitable forthe expansion process than others, since they can be processed over awider range of stretching rate and temperature. The primary requisite ofa suitable resin is a very high degree of crystallinity, preferably in,the range of 98% or above.

The porous microstructure of the expanded material is affected by thetemperature and the rate at which it is expanded. The structure consistsof nodes interconnected by very small fibrils. In the case of uni-axialexpansion the nodes are elongated, the longer axis of a node beingoriented perpendicular to the direction of expansion. The fibrils thatinterconnect the nodes are oriented parallel to the direction ofexpansion. The nodes may vary in size, depending on the conditions usedin the expansion. Products which have been expanded at high temperaturesand high rates have a more homogeneous structure, i.e., they havesmaller and more closely spaced nodes, and these nodes areinterconnected with a greater number of fibrils. These products are alsofound to have much greater strength. The expansion process results in atremendous increase in the tensile strength of the PTFE fibers and anincrease in the porosity.

When the expanded products are heated to a temperature above the lowestcrystalline melting point of the PTFE, disorder begins to occur in thegeometric order of the crystallites and the crystallinity decreases,therefore increasing the amorphous content of the polymer, typically to10% or more. These amorphous regions within the crystalline structureappear to greatly inhibit slippage along the crystalline axis of thecrystallite and appear to lock fibrils and crystallites so that theyresist slippage under stress. Therefore, the heat treatment may beconsidered an amorphous locking process. The important aspect ofamorphous locking is that an increase in amorphous content occurs,regardless of the crystallinity of the resin at start. When the materialis heated above 327° C. a surprising increase in strengths occurs.

The increase in strength of the polymer matrix is dependent upon thestrength of the extruded material before expansion, the degree ofcrystallinity of the polymer, the rate and temperature at which theexpansion is performed, and amorphous locking. When all these factorsare employed to maximize the strength of the material, tensile strengthsof 10,000 psi and above, with porosity of 90% or more are obtained. Incontrast, the maximum tensile strength of conventional extruded ormolded PTFE after sintering is generally considered to be about 3,000psi, and for conventional extruded and calendered PTFE tape, which hasbeen centered, the maximum is about 5,100 psi.

The prior art in dental floss, as exemplified by the U.S. Pat. Nos.3,830,246, 3,897,795, 4,215,478 and 4,033,365, is made of synthetic ornatural material, PTFE is not mentioned.

These patents show that flossing is an extremely important adjunct toproper dental hygiene. The insufficient consumer acceptance, despiteoften repeat directions by dentists to use floss, may arise from thefact that prior art flosses frequently caused gingival bleeding and aregenerally uncomfortable or difficult to use. Those conditions may ariseprimarily from the relatively high coefficient of friction (COF) of suchflosses.

Thus, because prior art flosses have such high coefficients of friction,consumers must use substantial force to pull them between the teeth orso-called “contact points”. Unfortunately, the user does not know whenthe floss will, in fact, pass between the contact points. When thissuddenly occurs, the user does not have time to release the great forcebeing applied. This appears to cause the flosses to be pulled into thegum, causing cuts that bleed, sometimes profusely. Hence, many of thedental flosses presently on the market have received limited consumeracceptance. The lack of consumer acceptance of any single dental flosson the market is due, in part, to the propensity of dental floss tocause gingival bleeding. In addition, dental floss is generallyconsidered difficult and uncomfortable to use. The consumerdissatisfaction with some dental flosses is caused by the relativelyhigh coefficient of friction.

In order to solve this problem a new type of dental floss made from PTFEhas become available from a variety of sources. This type of dentalfloss has certain beneficial characteristics, including high lubricityand a lower fraying rate than conventional flosses. Some patents havebeen aimed at such products including U.S. Pat. Nos. 5,033,488 and5,209,251 to Curtis et al., and U.S. Pat. No. 5,220,932 to Blass.

The Curtis patents (U.S. Pat. Nos. 5,033,488 and 5,209,251) disclose theuse of high strength expanded PTFE, which is coated with a material toincrease the PTFE coefficient of friction for use as a dental floss.

The Blass patent (U.S. Pat. No. 5,220,932) discloses the use of auni-axially stretched, non-porous PTFE having a relatively low tensilestrength, and is coated with wax to increase the PTFE coefficient offriction for use as a dental floss.

Expanded PTFE has a rather low coefficient of friction (below 0.08)compared to a coefficient of friction of about 0.2 for prior artcommercial flosses. The inventors in U.S. Pat. No. 5,033,488 show thatmicrocrystalline wax (MCW) adheres to expanded PTFE and, unexpectedly,provides a coefficient of friction sufficiently high to permit the userto securely grasp the floss and tapes, but generally not so high as thatof the prior art. This coefficient of friction is intermediate betweenthe very low coefficient of friction of expanded PTFE (below 0.08) andcoefficient of friction of commercial flosses, say about 0.08 and 0.15.

Other patents, such as U.S. Pat. Nos. 5,657,779 and 5,806,539 teach amethod for producing PTFE dental floss, comprising the passage of anunsintered PTFE tape across a heated surface in sliding contacttherewith, while applying tension to the tape, wherein the temperatureof the heated surface, the passage speed of the tape and the tensionapplied are such that the PTFE tape, when its temperature is raised bycontact with the heated surface, is longitudinally stretched. The dentalfloss produced comprises a PTFE tape having opposite faces at which therespective physical states of the PTFE material differ, the coefficientof friction being one of these differences. The opposite faces of thisdental floss have different degrees of sintering.

The U.S. Pat. No. 5,698,300 to Lenzing discloses a film consisting oftwo-or more PTFE layers which differ in their shrink propertiesproviding a bi-component fiber, which can be transformed into a staplecrimp by heating to a temperature above 200+ C. This U.S. patent furtherrelates to a process for producing a bi-component film from two types ofPTFE. The resultant film shrinks to different extents under the effectof heat and differs in Its hot-air shrinkage by at least 1%. Each typeof PTFE is molded in a cylinder and one half is joined to the other halfwith the other type of PTFE and then extruded, calendered, dried,sintered and cut into a staple fiber.

The U.S. Pat. No. 5,804,290 by Lenzing describes a dental floss thatcontains PTFE and whiting filler, and preserves the gum more than theprior art. By adjusting the amount of whiting material, the kineticfriction of the dental floss can be modified. Experiments have disclosedthat dental floss containing whiting from 0.1 to 15% by weight isparticularly well suited.

The U.S. Pat. No. 6,220,256 also discloses a dental floss made of PTFEand filler, this filler being fumed silica. The dental floss can have aplurality of layers of PTFE, with at least one of the layers havingfumed silica placed within it. Preferably, the filament has an innerlayer and two outer layers, with the fumed silica situated in at leastone of the two outer layers. The layers are made separately and thenlaminated by any conventional lamination technique, such as calenderingtogether with use of rotating rollers. The floss obtained through thisdocument provides increased surface friction.

In view of the above, it is an objective of the present invention toprovide a bi-color bi-component expanded PTFE dental floss where eachside presents different coefficients of friction.

Another objective of the present invention relies on providing adental-floss that permits the consumers to choose the side that theyintend to use.

Another objective of the present invention relies on providing a dentalfloss with sides of different colors to aid identification of the sidethat is more or less slippery.

Another objective of the present invention relies on providing afilament with sides having different coefficients of friction for otherapplications besides dental floss.

Accordingly, an inventive process for manufacturing a PTFE filament ofthe type comprising steps of extrusion, and, subsequently, stretching,heating and cutting, and an inventive PTFE filament obtained by thisprocess, are provided.

The process of the present invention comprises the following steps priorto extrusion:

providing a recipient, which preferably consists of a cylinder of are-form machine, having rigid side walls;

arranging a first mixture containing PTFE and a filler, and a secondmixture containing PTFE, inside the recipient, side by side and alignedwith the side walls; and

pressing the first and second mixtures in a direction parallel to theside walls to form a billet in which the first and second mixtures havedifferent coefficients of friction.

After these steps, the billet having the first and second mixtures isextruded to form a strand, which is subsequently stretched, heated,rolled and cut by known processes in the art to form the inventive PTFEfilament.

The filler has the purpose of providing a different coefficient offriction on each side of the PTFE filament. Accordingly, the coefficientof friction on the side with filler preferably ranging from 0.08 to0.20, and the side without filler being less than 0.08. The filler canbe made of silica, alumina, mica and/or calcium carbonate, among othercomponents.

Although, in a preferred embodiment, the second mixture does not containsuch a filler, other embodiments of the present invention can present afirst mixture with a first filler and a second mixture with a secondfiller, provided that the billet formed includes first and secondmixtures having different coefficients of friction.

In addition, unlike prior arts, the first and second mixtures may alsohave the same shrink properties, as the above difference in thecoefficient of friction between the mixtures is provided by the filler.

In an embodiment of the present process, different pigments made oforganic and inorganic materials may be mixed with the mixtures so thateach mixture can have a different color.

In the above or any other embodiment of the present invention, the stepof arranging can also comprise a step of inserting a barrier, preferablya plate, in the recipient to separate it into two-portions. Although, ina preferred embodiment, this barrier is a plate, other embodiments forthe barrier can be used without affecting the scope of the presentinvention. Then, the first and the second mixtures are respectivelyinserted into these two portions of the recipient. Subsequently, thisbarrier is removed, enabling a part of the first mixture to contact apart of the second mixture and be arranged by side by side and alignedwith the side walls of the recipient.

The PTFE filament obtained comprises one side with a filler and anotherside without filler, so that these sides have different coefficients offriction, wherein preferably the side with filler presents a highercoefficient of friction than the other side. Also, in a preferredembodiment, the filament presents each side with a different color.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plate being inserted into a cylinderof a pre-form machine according to the preferred embodiment of thepresent invention.

FIG. 2 is a perspective view of the cylinder including the plateaccording to the embodiment depicted in FIG. 1.

FIG. 3 is a perspective view of the cylinder including the plate andmixtures A and B according to the embodiment depicted in FIG. 1.

FIG. 4 is a perspective view of the plate being removed from thecylinder according to the embodiment depicted in FIG. 1.

FIG. 5 is a perspective view of a billet formed after the mixtures A andB are pressed in the cylinder according to the embodiment depicted inFIG. 1.

FIG. 6 is a perspective view of a strand formed after the billet isextruded according to the embodiment depicted in. FIG. 1.

FIG. 7 is a perspective view of a PTFE filament according to theembodiment depicted in FIG. 1.

DETAILED DESCRIPTION

The invention will now be described in further detail on the basis oftests and examples.

Steps in the process:

(a) Mixture/Paste-Extrusion/Tape

Two mixtures, A and B, are produced in the following manner:

A—A fine-powder PTFE resin is pre-mixed with silica or alumina fillerand then a liquid lubricant is added until a compound is formed.

B—A fine powder PTFE resin is mixed with a liquid lubricant, until acompound is formed.

The first and the second mixtures preferentially have the same shrinkproperties, and, furthermore, in one or both mixtures a pigment is addedfor identification of the compositions.

The volume of lubricant used in these mixtures should be sufficient tolubricate the primary particles of the PTFE resin so as to minimize thepossibility of shearing of the particles prior to extruding. Theproportion ranges from 17% to 29%. In mixtures, the amount of filler canvary from 0% to 25% and the amount of pigment ranges from 0% to 10%.These mixtures are processed, preferably for 20 to 30 minutes.

Other kinds of filler besides silica and alumina can be used in thecompound and the pigments can be organic or inorganic.

As indicated in FIGS. 1 to 5, a cylinder of a pre-forrm machine 1 ispreviously axially split along its length into two halves by a plate 2or strip and then the mixture A is fed into one half and the mixture Binto the other half. After feeding the cylinder 1 with the differentcompounds, the plate 2 that separates the same into equal parts is takenout and the material is preformed. Consequently, a bi-color bi-componentbillet 5 is formed, wherein one half comprises mixture A and the othercomprises mixture B, and, furthermore, presenting excellent adhesion atthe interface of the mixtures.

After these steps, the billet 5 having the mixtures A and B is extrudedto form a strand 6, as depicted in FIG. 6, which is subsequently rolled,stretched, heated and cut by known processes in the art to form theinventive PTFE filament (FIG. 7), as follows.

In the extrusion process, a reduction ratio of about 10:1 to 1000:1 maybe used (i.e. reduction ratio=cross-sectional area of extrusion cylinderdivided by cross-sectional area of the extrusion die). For mostapplications, a reduction ratio of 25:1 to 200:1 is preferred.

The strand is, in the next stage, pressed through calender rolls inorder to form a tape with a thickness ranging from 50 μm to 1000 μm. Inthis method, care should be taken that the strand runs in between therollers such that the imaginary separating line of the two materialhalves lies parallel to the roller nip. Then a bi-color bi-componenttape is produced, where one side consists of PTFE-filler and the otherside is composed of pure PTFE. These sides can be of a variety ofcolors. The tape resulting from the calendering, with one side one colorand the other side another color, passes through a drying oven to removethe liquid lubricant. The drying temperature ranges from 100° C. to 300°C.

(b) Stretching and Heat Treatment

In this invention, it has been found that such composite tape can beexpanded by stretching in at least one direction about 1.1 to 100 timesits original length (with about 2 to 50 times being preferred). Thestretching is carried out by passing the dry composite tape throughtensioning rollers between the two units of pulling rollers that operatewith a stretching ratio that is, the ratio between the entry speed andthe exit speed—from 1.1 to 100, and a stretching temperature rangingfrom 150 to 300° C. The expanded PTFE composite tape can be optionallylongitudinally expanded further if desired. The heat element in theexpansion process may be an oven, a hot-air, steam or high-boiling-pointliquid heater, a heated plate or a heated cylinder.

After the stretching, the composite tape is wound in a winder.

The tape may be formed into filaments by slitting the expanded compositetape into pre-determined widths (between 0.5 to 10 mm), feeding it intothe cutting unit, whereby the individual PTFE filaments are cut andseparated.

Following cutting, the composite expanded PTFE filaments, where one sideis composed of PTFE-filler (one color) and the other side of pure PTFE(another color), may then be further stretched. The composite filamentis again stretched with a stretching ratio ranging from 1.1 to 20 (with1.2 to 8.0 being preferred) under high temperature (between 300 to 450°C.) in order to subject the fiber to an amorphous locking step. Thestretched filaments are wound individually in the winding unit.

The expanded PTFE filament obtained from the technique described aboveis depicted in FIG. 7, reference number 7, and presents two sides withdifferent characteristics, principally their coefficients of friction,determined according to the method described below. On side A, thefilament contains PTFE-filler (one color) and, on side, B, contains purePTFE (another color).

The final characteristics of the dental floss comprise: a width of about0.5 to 3.0 mm; a thickness of about 20 to 400 μm; a weight/length ofabout 400 to 2000 dtex; a density of about 0.7 to 2.2 g/cm³; a tensilestrength ranging from 100 to 1100 MPa and a tenacity ranging from 2.0 to6.0 cN/dtex. The coefficient of friction on both sides are different,the side with filler ranging from 0.08 to 0.20 and the side withoutfiller being less than 0.08.

Each of these properties is measured in the following manner length,width and thickness are determined through the use of calipers; densityby dividing the measured weight of the sample by the computed volume ofthe sample; the volume is computed by multiplying the measured length,width and thickness of the sample.

The bulk tensile strength of the fibers is measured by a tensile tester,such as an INSTRON Machine by using the following conditions. The gaugelength is 250 mm and the cross-head speed of the tensile tester is 250mm/min.

Tenacity is calculated by dividing the maximum force obtained in thetensile tester by its normalized weight per unit length (tex(grams/1,000 meters) or dtex (grams/10,000 meters) or denier(grams/9,000 meters)).

The coefficient of friction is a dimensionless quality which representsthe force required to move an object across a surface. This test methodcovers the measurement of kinetic friction between fiber and solidsurfaces of a constant radius in the contact area. In general, thegreater the value of the coefficient of friction, the more difficult itis to move the object with respect to the surface, and thus, a greaterfrictional force is involved. Various properties of dental floss can beinferred from coefficient of friction experiments, such as ease ofinter-proximal access and gentleness of the floss on gingival tissue.

Apparatus required for determination of the coefficient of friction:

Instron Machine, friction testing apparatus—with rotating mandrels and100 g weight.

Procedure:

Preset the Instron with the following parameters:

Cross-head weight—5 Kg

Cross-head speed—190 mm/min

Gauge length—110 mm

Reference weight—100 g

Angle of wrap—240=4.189 rad

Recorder speed—5 cm/min

(1) Measure 5 pieces of floss, each 110 mm in length

(2) Attach one strand of floss to upper cross-head. Let it hang betweenthe mandrels, not touching them.

(3) Place a 100 g weight at the other end of the sample.

(4) Measure the force recorded by the recorder resulting from the weightand the floss sample. When the floss and weight are raised by theInstron unit, this value remains constant. This value is the restingweight

(5) To measure the coefficient of friction, wrap the floss sample aroundtwo mandrels

(6) Make sure that the floss is not twisted and is very steady

(7) Zero the chart recorder and start

(8) Start the mandrels rotating

(9) Press the start button to start raising the floss over the rotatingmandrels

(10) Let the instron raise the floss to approximately 3 inches from the100 g weight

(11) Select 10 peaks from chart recorder and average this data

(12) Perform calculation below on average this data:

coefficient of friction=( 1/0)*In(T2/T1)

Where:

coefficient of friction=coefficient of friction

0=angle of wrap in radius

T2=tension while system is being pulled over mandrels

T1=tension in floss sample+weight at rest

(13) Repeat steps 5-12, four more times, using a new floss sample everytime.

A few examples will be described hereinafter on the basis of testsperformed under different conditions:

EXAMPLE 1

A bi-color bi-component dental floss of the present invention isproduced as described below:

Two mixtures, A and B, are produced in the following manner

A—A fine-powder PTFE resin is pre-mixed with quartz silica filler andthen a liquid lubricant is added until a compound is formed.

B—A fine-powder PTFE resin is pre-mixed with an organic pigment and thena liquid lubricant is also added until a compound is formed.

These mixtures are processed preferably for 20 to 30 minutes and shouldhave the same pressure extrusion. The proportion of lubricant in thesemixtures ranges from 17% to 29%. In mixture A, the amount of quartzsilica is 4.7% and in the mixture B the amount of pigment is 0.5%.

The cylinder of a pre form machine 1 is fed with the mixture A in onehalf and the mixture B in another half (FIG. 3). After feeding thecylinder 1 with the different compounds, the plate 2 that separates thesame into equal parts is removed and the material is pre-formed (FIG.4). Consequently, a bi-color bi-component billet 5 is formed, whereinone half comprises mixture A and the other comprises mixture B (FIG. 5).

After these steps, this billet is extruded to form a strand 6 (FIG. 6),which is subsequently rolled, stretched, heated and cut by knownprocesses in the art to form the inventive PTFE filament, as follows.

A reduction ratio of 148:1 is used. The bi-color bi-component taperesulting from calender rolling, one side of which is white (PTFE-quartzsilica) and the other colored side (PTFE-pigment), with a thickness of400 μm. This tape is passed through an oven at a temperature of 220° C.for lubricant removal. The dry tape is stretched uni-axially in thelongitudinal direction 8.0 times its original length by passing the drytape through tensioning rollers between the two units of pulling rollersthat operate with a stretching ratio of 8.0 and a stretching temperatureof 265° C.

The expanded tape is slit to 3.0 mm widths by passing it between a setof gapped blades. The slit strands are further stretched uniaxially inthe longitudinal direction over hot plates at a temperature of 400° C.and at a ratio of 4.0 to form a bi-color bi-component dental floss.

The following measures are taken on the bi-color bi-component expandedPTFE dental floss:

Filament Number: 850 dtex Tensile Strength: 420 MPa Tenacity: 3.3 g/dtexCoefficient of friction: White side 0.08 Colored side 0.06

EXAMPLE 2

A bi-component dental floss is produced, as described in Example 1. Thedifference is in the composition of the mixtures. The proportion ofquartz silica in the white side is 12.3%.

The following measures are taken on the bi-color bi-component expandedPTFE dental floss:

Filament Number: 870 dtex Tensile Strength: 340 MPa Tenacity: 2.8 g/dtexCoefficient of friction: White side 0.10 Colored side 0.06

EXAMPLE 3

A bi-component dental floss is also produced, as described in Example 1.In this case, the type of filler used is precipitated silica (whiteside) in the proportion of 8.2%.

The following measures are taken on the bi-color bi-component expandedPTFE dental floss:

Filament Number: 860 dtex Tensile Strength: 400 MPa Tenacity: 3.1 g/dtexCoefficient of friction: White side 0.10 Colored side 0.06

EXAMPLE 4

A bi-component dental floss is also produced, as described in Example 1.Alumina is the filler, used in the proportion 12%.

The following measures are taken on the bi-color bi-component expandedPTFE dental floss:

Filament Number: 870 dtex Tensile Strength: 350 MPa Tenacity: 2.8 g/dtexCoefficient of friction: White side 0.09 Colored side 0.06

Although described in connection with specific examples, the presentinvention is not intended to be limited thereto, but rather includessuch modifications and variations as are within the scope of theappended claims.

1. A process for manufacturing a PTFE filament of the type comprisingsteps of extrusion, and, subsequently, stretching, heating and cuttingPTFE, characterized by the following steps prior to extrusion: providinga recipient having rigid side walls; arranging a first mixturecontaining PTFE and a filler, and a second mixture containing PTFE,inside the recipient, side by side and aligned with the side walls; andpressing the first and second mixtures in a direction parallel to theside walls to form a billet in which the first and second mixtures havedifferent coefficients of friction.
 2. The process according to claim 1is characterized by the fact that, in the arranging step, the first andthe second mixtures are inserted respectively into two portions of therecipient separated by a barrier, and, subsequently, the barrier isremoved, enabling a part of the first mixture to contact a part of thesecond, and be arranged side by side and aligned with the side walls ofthe recipient.
 3. The process according to claim 1 or 2 is characterizedby the fact that, in the step of arranging, the first mixture includes apigment and the second mixture includes another pigment.
 4. A PTFEfilament obtained by the process defined in claim 1 is characterized bycomprising one side with a filler, so that this side has a differentcoefficient of friction in relation to the other side.
 5. The PTFEfilament in claim 4 is characterized by the fact that the first and thesecond mixtures have the same shrink properties.
 6. The PTFE filament inclaim 4 or 5 is characterized by further comprising a lubricant.
 7. ThePTFE filament in any one of claims 4 to 6 is characterized by the factthat each side has a different color.
 8. The PTFE filament in any one ofclaims 4 to 7 is characterized by the fact that the filler comprises atleast one of silica, alumina, mica and calcium carbonate.
 9. The PTFEfilament according to any one of claims 4 to 8 is characterized by thefact that the quantity of filler in the respective side ranges from 1 to25%.
 10. The PTFE filament according to any one of claims 4 to 9 ischaracterized by the fact that the quantity of pigment in at least oneside ranges from 0.05% to 10%.
 11. The PTFE filament according to anyone of claims 4 to 10 is characterized by the fact that said coefficientof friction in the side with filler ranges from 0.08 to 0.20 and theother side is less than 0.08.
 12. The PTFE filament according to any oneof claims 4 to 11, characterized by comprising a width ranging from 0.5to 3.0 mm, a thickness ranging from 20 to 400 μm, a density ranging from0.7 to 2.2 g/cm3, a tensile strength ranging from 100 to 1100 MPa and atenacity ranging from 2.0 to 6.0 cN/dtex.